The inhibitory and stimulatory neuroendocrine processes that control prolactin secretion in rhesus monkeys will be studied. The extrahypothalamic and hypothalamic sites that influence basal secretion and diurnal pattern in serum prolactin will be identified. Specific neurochemicals with prolactin-inhibiting (PI) and prolactin-stimulating (PR) activities and their possible sites of action will be evaluated by means of the techniques of neural disconnections, local central nervous system lesions, electrical stimulation in discrete neural regions, indwelling cranial cannulae for neural peptide and drug infusions, and pituitary-stalk transections. Changes in serum prolactin will be measured by radioimmunoassay. The influence of steroid hormones, particularly estrogen, on prolactin secretion and their possible sites of action in the regulation of PI and PR neurochemicals will be studied.